1. Field of the Invention
This invention generally relates to integrated circuit (IC) and semiconductor processes and, more particularly, to a germanium (Ge) short wavelength infrared (SWIR) imager and associated fabrication process.
2. Description of the Related Art
Short wavelength infrared (SWIR) imagers, with about a 1 to 1.7 micron wavelength, are used in camera systems for military and security night vision applications, as well as in many machine vision applications. Most conventional SWIR light-sensitive components are made of InGaAs films epitaxially grown on InP substrates. Compared to Si substrates, InP substrates are small (only up to 4 inches in diameter), fragile, and very expensive.
By adjusting the relative In and Ga content, the lattice constant of InGaAs can be tailored to match the InP lattice constant, or be very close to it, thus minimizing threading dislocations and the accompanying increase in dark current. An array of diodes is typically formed in the InGaAs film, to form a focal plane array of pixels. Each diode is typically 25-40 microns square. The diodes may be isolated by trenches filled with dielectric, or by a guard-ring implant. Next, the two terminals of each InGaAs diode have indium contacts formed on them. The InGaAs/InP assembly with In contacts is then flip-chip bonded to a readout circuit typically fabricated using Si CMOS technology. To minimize the noise due to dark current, the readout circuit is often designed to keep the bias across each InGaAs diode at close to zero volts. A variety of circuits are used, but the best usually use op-amps in the interface. Since the InGaAs pixels are large (25 microns or more) this is not a problem for current Si CMOS technology. After flip-chip bonding, the incident SWIR light has to travel through the InP substrate to get to the InGaAs film. However, InP is mostly transparent at these wavelengths, so the light travels through the InP substrate without significant absorption and reaches the InGaAs film, where it is absorbed, turned into an electrical signal, and fed to the read-out circuit, from which it is turned into a digital image.
Therefore, the good SWIR absorption characteristics of InGaAs are combined with the reliability and low cost of Si CMOS readout circuits. However, due to the small size and fragility of available InP substrates, the cost of fabricating epitaxial InGaAs films on InP substrates is very high. This, in turn, keeps the cost of cameras based on these image sensors very expensive. Nevertheless, this technology has found good acceptance in high-end applications, such as night vision cameras for security and military use and machine vision cameras for inspecting products ranging from apples, to pharmaceuticals, to Si integrated circuits.
Due to the large lattice mismatch between Ge and Si (4.2%), it is not easy to obtain Ge films on Si with proper flatness and low defect density. However, in recent years a variety of methods have been developed to produce good quality Ge films on Si substrates. One early method uses the ultrahigh vacuum chemical vapor deposition (UHVCVD) growth of Ge on relaxed, graded SiGe buffer layers. This results in dark (leakage) currents in p-i-n diodes as low as 0.15 mA/cm2 at 1V reverse bias, which is just a few times higher than the theoretical limit of ˜0.05 mA/cm2 for their device structure. However, this method requires a SiGe buffer over 10 microns thick and a CMP step to reduce surface roughness. Similar dark current results have recently been obtained with Ge on a 4 micron thick SiGe buffer grown using Sb surfactant-mediated MBE. Another technique is to first deposit Ge at ˜350° C. and then at ˜600° C. This two-step process results in micron-thick Ge films with smooth surfaces, without a CMP step. This method can be combined with the technique of cycle annealing to concentrate the threading dislocations near the Ge/Si interface, and so reduce the leakage currents. Using similar techniques, several groups have fabricated near-infrared Ge photodetectors for telecommunication applications. Typically, the dark current in these devices is 10-100 mA/cm2 at 1 V reverse bias. We have also demonstrated the advantage of two-step growth followed by cycle annealing, achieving dark currents as low as 5 mA/cm2 at 1V bias in 1.5-2 micron thick Ge films.
Two-step epi-Ge growth followed by cycle annealing can be used to fabricate back-side illuminated Ge photodetectors using SOI substrates. Si wafers are transparent to wavelengths longer than 1100 nm, so light between 1100 nm and 1600 nm, which is incident on the backside of the Si wafer, can travel through the wafer to be absorbed by the Ge film. The buried oxide (BOX), silicon-on-insulator (SOI), and Ge thicknesses can be chosen to produce a resonant cavity at the wavelength of interest, 1550 nm. In this way, the photodetection is enhanced.
An alternative to growing Ge directly on Si is to form a Ge film by using one of a variety of wafer bonding techniques. Nearly perfect bulk Ge wafers are now available in sizes up to six inches, but the wafers are more fragile than Si. The advantage is that these bonded Ge films should exhibit fewer defects and lower dark current. The disadvantage is that the method is limited by the size of available Ge wafers.
All of the above-mentioned techniques still produce Ge films which have higher leakage currents than epitaxial InGaAs/InP, since even perfect Ge has intrinsically higher leakage than perfect InGaAs. However, if operated at zero bias, the dark current of a diode is ideally zero. So, if Ge diodes can be operated at sufficiently close to zero volts, even imperfect, leaky material can be used to make imagers having noise low enough for many applications.
It would be advantageous to use a Ge film on Si substrate as a SWIR light-sensitive component, as Si substrates are large (currently up to 12 inches in diameter) and robust.